The invention relates to a method and apparatus for digitally demodulating an analog signal comprising a linearly-modulated carrier and representing successive symbols. An important application for the invention lies in demodulators of modems that satisfy CCITT recommendations V27 and V29. The invention is nevertheless also applicable to test equipement and to equipment designed to demodulate and concentrate a plurality of modem calls.
Before describing the invention, it is appropriate to recall the fundamentals of linearly modulating a carrier to represent symbols and the currently used demodulation methods.
Linear modulation makes it possible for a signal to represent a digital sequence constituted by n different symbols a.sub.1, . . . , a.sub.n by means of an alphabet S.sub.1 (t), . . . , S.sub.n (t). Each of the elementary signals constituting the alphabet is zero outside a time interval [0,T], where 1/T is the symbol tranmsission rate. A symbol a.sub.p of order p in the alphabet of symbols is transmitted in time interval [kT,(k+1)T] and is represented as S.sub.p (t-kT).
The kinds of linear modulation that are most used are phase modulation, amplitude modulation, and combined phase and amplitude modulation.
For a rate of 4800 bits/s, CCITT recommendations V29 and V27 respectively specify four-state phase modulation and eight-state differential phase modulation, known as PSK. For this type of modulation, the symbols are represented by: EQU S.sub.p (t)=A.multidot.cos(2.pi.f.sub.o t+.phi..sub.p)
where A is a constant amplitude;
f.sub.0 is the carrier frequency;
and .phi..sub.p is a phase which takes n different values in the range 0 to 2.pi..
With combined phase and amplitude modulation, the symbols a.sub.p are represented by: EQU S.sub.p (t)=A.sub.p (2.pi.f.sub.0 t+.phi..sub.p)
where, for two different symbols a.sub.p and a.sub.q : EQU (A.sub.p, .phi..sub.p).noteq.(A.sub.q, .phi..sub.q)
CCITT recommendation V29 specifies such modulation for the 9600 bits/s rate (quadrature amplitude modulation, QAM).
Since the phase absolute value is unknown at the receiver, demodulation is performed differentially : the symbol transmitted in interval [kT, (k+1)T] is assigned to the phase difference between the signal transmitted in said interval and the signal transmitted during the preceding interval [(k-1)T, kT].
The pass band of the transmission channel is generally limited. Consequently, filtering is required. In general, Nyquist filtering is used to eliminate intersymbol interference (ISI) at instants which follow one another at the symbol transmission rate. These symbols are then identified by determining the complex envelope of the received and filtered signal, sampling at instants when there is no ISI, and then comparing with thresholds or implementing some other decision argorithm.
A common solution consists in sharing Nyquist filtering equally between transmission and reception : each of the transmitter and the receiver includes irrespective filter having a transfer function such that its product with the transfer function of the other filter constitutes a raised cosine Nyquist filter.
Proposals have already been made for a digital method of demodulating an analog signal comprising a linearly modulated carrier representing successive symbols, the method comprising filtering to reduce inter-symbol interference, sampling at a frequency that is at least twice the symbol transmission rate, detecting the complex envelope for eliminating the carrier, and decoding to recover the symbols. Document U.S. Pat. No. 4,800,574 describes a demodulator for implementing such a method and in which all operations are digital, except for the input filtering to reduce the noise caused by the limited pass band of the channel: to this end, the output signal from the filter is applied to an analog-to-digital converter. A demodulator organized in that way cannot provide complete demodulation of a plurality of simultaneous communications.
A receiver for receiving data from a recorded PSK or ASK signal has also been described in FR-A-2566604. It comprises an input analog-to-digital converter for sampling and digitizing the signal and two filters in quadrature. The clock which defines the sampling times is controlled by a loop for tracking instabilities of the record, whereby synchronism between the sampling frequency and the data rate may be achieved. The filters may further be controlled for adjusting the propagation time therein and to compensate for fast rate changes within a limited range, thereby solving a problem which is specific to demodulation of data stored on a medium, such as a magnetic tape, exhibiting unstabilities, however without loss of synchronism between the carrier and the rate signal.